Yes you can put windows in, but...
I posted a World Cab parts book some time ago in this thread:
https://www.cj-8.com/forum/showthread.php?t=16400
The Postals are Van tops and the Aussie ones are Wagon tops. Check out pages 19 and 20 of the PDF in that thread. You'll see that the Van and Wagon have two different part numbers for the inner side panel - Van = 2012 and 2014, Wagon = 2016 and 2018 (reference numbers 11 and 17 in the drawing).
The difference between the two is the size of the window opening in the panel, the place where you'd cut the opening in the Van panel is smaller than the Wagon window opening.
So the answer is you can cut a hole in your Postal top and put a window in, but the stock Aussie Wagon window with the pop-outs won't fit. You'll have to find a different window, perhaps at an RV supply place.
If you do decide to cut the hole in the Postal top, you'll also have to deal with the horizontal reinforcing rib in the inner panel. You'll need to cut that out and then where the rib used to join the vertical part of the window opening you'll need to weld in some patch panels to match the contour of the window frame, these patch panels can be cut from the reinforcing rib. I have detailed photos of that if you need them, let me know and I'll post them. The basics of the window cutouts are shown on my project site:
http://home.stny.rr.com/jscherb/scrambler/hardtop.htm. Notice my side windows are smaller than the ones in the photos you posted, that illustrates the difference between the size of the window openings on the inner panels. Oh and you'll probably have to deal with the vertical curve of the side panel.
If you want some detailed photos of cutting the window opening in a Postal top and dealing with the curved side so a flat window can fit in, let me know and I'll post them.
jeff
PS: The window parts are on pages 17 and 18 of the same PDF.